More people than ever are using wireless communication devices (WCDs), such as cellular telephones and personal digital assistants (PDAs), to connect to cellular wireless communication systems and other sorts of wireless networks. Through such networks, a mobile station may conveniently engage in voice and/or packet-data communications using a designated air interface protocol, such as CDMA (e.g., 1xRTT, 1xEV-DO, or 1xEV-DV), TDMA, GSM, AMPS, WiMAX, 802.11, BLUETOOTH, or any other protocol now known or later developed.
In general, a cellular wireless communication system operated by a wireless service provider or wireless carrier provides one or more radio access networks (RANs) that each include infrastructure arranged to facilitate wireless communication by served WCDs. A typical RAN includes at least one base transceiver station (BTS) comprising one or more antennas that radiate to define a cell, and perhaps cell sectors, in which WCDs can communicate with the RAN through a radio frequency (RF) air interface.
Each BTS may then be coupled with a base station controller (BSC) or similar entity that controls functions of the BTS and manages air interface functions such as handoff between sectors or cells for instance. Further, each BSC may then be coupled with a switch or gateway that provides connectivity with a transport network. For instance, the BSC may be coupled with a mobile switching center (MSC) or similar entity that provides connectivity with the public switched telephone network (PSTN), to allow suitably equipped WCDs to place and receive PSTN telephone calls. Likewise, the BSC may be coupled with a packet data gateway that provides connectivity with a packet-switched network such as the Internet, to allow suitably equipped WCDs to engage in packet-data communication.
Various different communication protocols well known to those of ordinary skill in the art define procedures for interaction between a WCD and a RAN, to facilitate management of air interface communications and management of calls or data sessions. For instance, protocols define how the air interface may be divided into control channels for control communication between the WCD and the RAN, and traffic channels for carrying bearer communication between the WCD and the RAN. Further, protocols define how calls (whether PSTN calls or data communications) can be set up to or from the WCD via the RAN.
Under the well known CDMA protocol, as defined by the CDMA2000 standard published by the Telecommunications Industry Association for instance, when a WCD first powers on or enters into coverage area of a RAN, the RAN registers with the RAN by sending a registration control message over the air to the RAN. The RAN then signals in turn with a home location register to record the presence of the WCD in the RAN, so that incoming calls can be routed to the WCD via the RAN. Thereafter, to initiate an outgoing communication via the RAN, the WCD may send a call origination message over the air to the RAN. The BSC of the RAN may then assign an air interface traffic channel for use by the WCD, and an MSC or packet-data gateway may set up the call via an appropriate transport network. Likewise, when a RAN receives a request to set up a call to a WCD, the RAN may send a page message over the air to the WCD, receive a page response over the air from the WCD, assign a traffic channel for use by the WCD, and send an alert/ring directive over the air to the WCD to cause the WCD to ring. When a user of the WCD answers the call, the WCD may then signal over the air to the RAN, and the RAN may connect the call through to the WCD.
Typically, each wireless coverage area (e.g., sector) in a RAN broadcasts a pilot signal over the air for receipt by WCDs, to enable WCDs to select one or more wireless coverage areas in which to operate. Under CDMA, for instance, a base station broadcasts a unique pilot signal in each of its sectors. Each served WCD then regularly scans the airwaves for pilot signals and measures the signal strength (e.g., ratio of energy to spectral interference, or Ec/Io) of each received pilot signal. When the WCD detects a new pilot signal that is sufficiently stronger than the pilot signal of a wireless coverage area in which the WCD is currently operating, the WCD then automatically sends a reporting signal over the air to the RAN, to request a handoff to the new wireless coverage area.
Further, WCDs such as cell phones typically store a “preferred roaming list” (PRL), which includes a prioritized listing of wireless serving systems that the mobile station may access. A PRL enables a WCD to roam between wireless serving systems operated the WCD's wireless carrier as well as between wireless serving systems operated by competing wireless carriers.
As a general matter, a PRL can take any of a variety of forms. By way of example, a rudimentary PRL may take the form of a simple flat file listing or data table in which each entry corresponds with a respective system and indicates a radio frequency and system/network identifier for the system. Using such a PRL, a WCD may repeatedly sequence through the listed entries, for each entry scanning the indicated frequency in search of a pilot carrying the indicated identifier, and the WCD may register with the first (i.e., highest priority) system that it finds.
More typically, a PRL will include a separate “acquisition table” and “system table.” The acquisition table may list frequencies to be scanned for allowed systems, and the system table may then provide a priority-ordered listing of allowed systems, specifying for each system an associated system identifier and/or network identifier. Under CDMA, for instance, each system table entry usually specifies a respective “system identification code” (SID) and “network identification code” (NID), and each CDMA base station broadcasts its SID and NID in system overhead messages. Thus, when a WCD detects an allowed SID-and-NID combination, the mobile station may connect to the identified system.
A wireless carrier will typically load an initial PRL onto its served WCDs at the time of device activation and may subsequently update the PRL through over-the-air signaling with the WCD. As the carrier exerts control over the PRL, the carrier will typically define the PRL in a manner that favors WCD operation in the carrier's own systems rather than in competitor systems. Thus, when a WCD is physically located in an area where service is available from both its own carrier's system and a competitor's system, the PRL may cause the WCD to register in and thus operate in its own carrier's system. On the other hand, if the WCD roams into an area where its own carrier does not provide service or where the signal strength from the carrier's serving system is too low in accordance with business rules defined by the PRL, the PRL may enable the WCD to detect, register and operate in a competitor system.